Activated phosphors having matrices of yttrium-transition metal compound

ABSTRACT

A phosphor composition contains a lanthanide activator element within a host matrix having a transition element as a major component. The host matrix is composed of certain rare earth phosphates or vanadates such as YPO4 with a portion of the rare earth replaced with one or more of the transition elements. On Xray or other electromagnetic excitation, trace lanthanide impurities or additives within the phosphor are spectrometrically determined from their characteristic luminescence.

United States Patent 1 De Kalb et al. July 8, 1975 [54] ACTIVATEDPHOSPHORS HAVING 3,539,857 ll/l970 Shaffer 252/3OL4 P X TRI E F Y'ITRIUMTRANSITI N 3,586,634 6/1971 Avella 252/3014 P X mE SS 0 3,647,706 3/1972Lagos 252/3014 P [75] Inventors: gdwalrdbLihDeflialb; VIeImer A. PrimaryExaminer jack Cooper o 0 owa Attorney, Agent, or FirmJohn A. Horan;Arthur A. [73] Assignee: The United States of America as Churm; Hugh W,Glenn represented by the United States Energy Research and DevelopmentAdministration, Washington, DC. [57] ABSTRACT [22] Filed: Jam 4 1973 Aphosphor composition contains a lanthanide activator element within ahost matrix having a transition pp 321,064 element as a major component.The host matrix is composed of certain rare earth phosphates or vana- 52U.S. Cl. 252/3014 P dates such as (P04 with a Portion rare earth [511Int. Cl C09k 1/36 placed with one or more of the transition elements" 53Field of Search 252/3014 P x'ray or electromagnetic excitatmm lanthanideimpurities or additives within the phosphor [56] References Cited arespectrometrically determined from their charac- UNITED STATES PATENTS5/1966 Borchardt 252/30l.4 P

teristic luminescence.

2 Claims, 7 Drawing Figures MTEI'ITFDJUL 8 ms SHEET oood.

AllSNlLNI mrnamm 8 m5 9.999.999

sum 4 Th T YZCO3 (P04 )4 L. (D Z 3 5 WAVELENGTH (i) wsmgmm 8 ms 3.893;939

SHEEI 5 y I Y Mn (P0 Sm Tb D Eu l; Tb w z E Sm E Eu WAVELENGTH (A)mflgmsmm 8 I975 3, 893, 939 SHEET 6 Eu Gd Tb Sm h 2 I x u Sm 5 J: I I II 3,000 5,000 7,000

WAVELENGTH (5) Fig- NuYCu (P0 Tb Tb [T +D Tb Y 'ism INTENSITY Gd rWAVELENGTH (K ACTIVATED PHOSPHORS HAVING MATRICES OF YTTRIUM-TRANSITIONMETAL COMPOUND CONTRACTUAL ORIGIN OF THE INVENTION The inventiondescribed herein was made in the course of, or under, a contract withthe United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION Lanthanides and other rare earth elementsincluding yttrium are often added either as a mixture. that ismischmetal, or individually to improve the properties of various steelalloys. Metallurgists have found that rare earths in parts per millionquantities improve both the mechanical and plastic properties by actingas strong desulfurizing and deoxidizing agents. Present quality controlmethods for quantitatively determining the levels of rare earthadditives within steel products involve long and laborious separationand concentration steps. Prior attempts to employ X-ray luminescenttechniques have been unsuccessful due to the quenching effect impartedby iron and other transition metals.

It is particularly desirable to determine the amount of lanthanidespresent in alloys that are to be used in nuclear reactors and otherapplications involving neutron radiation. Various lanthanides, forinstance isotopes of neodymium, samarium, europium, gadolinium anddysprosium, have very large cross sections for thermal neutron captureand are therefore objectionable additives to construction materials infacilities where neutron economy is important.

Lanthanide impurities or additives have been included in a large numberof hosts other than the transition elements to produce phosphors thathave fluorescent characteristics. These phosphors along with samplespectra are presented in X-ray Excited Optical FluorescenceSpectrometry. Scope of Application to Trace Rare Earth Determinations byDeKalb, DSilva and Fassel, Analytical Chemistry, pp. l246-I25l, Vol. 42,No. l l, Sept. [970. Effective crystalline hosts are suggested with awide variety of cations forming simple, binary and ternary oxides.However, none of the hosts materials presented in the article includetransition elements from groups VIIB or VIII, particularly manganese,iron, cobalt and nickel. Moreover, copper from group I8 was found to bea noneffective host material in one particular mixture. For purposes ofthis application, the elements ofeach group are defined by the periodictable given in Daniels and Alberty, Physical Chemistry, p. 473, 2d. Ed.,John Wiley and Sons, Inc., New York, London, 1955, I96l.

SUMMARY OF THE INVENTION In view of the limitations of the prior art, itis therefore an object of the present invention to provide a phosphorcomposition including one or more transition elements as major hostcomponents in which trace amounts of lanthanides can be made toluminesce with characteristic spectra.

It is also an object to provide a phosphor composition including one ormore of the group of elements consisting of iron, nickel, cobalt,manganese and copper.

It is a further object of this invention to provide a method for thedetermination of lanthanide additives in transition element alloys thatdoes not require the chemical separation of these additives from thehost materials.

It is yet another object to provide a radiation interrogation techniquefor the determination of trace lanthanides in steel products Inaccordance with the present invention, a phosphor composition isprepared containing, as matrix host, the phosphates or the vanadates ofweak fluorescing rare earth elements, particularly yttrium, scandium,lanthanum, gadolinium or lutetium and of one or more of the transitionelements. The phosphor is activated by the inclusion of trace quantitiesof various lanthanides. On exposure to X-rays or other electromagneticradiation, the resulting luminescence is spectrometrically analyzed toquantitatively determine the trace lanthanides.

DESCRIPTION OF THE DRAWINGS FIG. I shows X-ray excited luminescentspectra for phosphors containing 303 Stainless Steel with variouslanthanide additives.

FIG. 2 shows X-ray excited spectra of phosphors containing iron.

FIG. 3 is a graph of spectra line intensity ratios versus lanthanideconcentration.

FIG. 4 is an X-ray excited spectrum for a phosphor containing cobalt.

FIG. 5 is an X-ray excited spectrum for a manganesecontaining phosphor.

FIG. 6 is a similar spectrum for a phosphor containing nickel.

FIG. 7 is a similar spectrum for a phosphor containing copper.

DETAILED DESCRIPTION OF THE INVENTION In preparing the phosphors of thepresent invention, acid solutions of the various elements are combined,followed by evaporation, dehydration and heating at appropriatetemperatures. It is preferred that the heating temperature be maintainedbelow that of the melting point of the lowest-melting component and thata sintering condition not be reached to obtain strong lu minescentspectra. The heated oxide can be subsequently ground to a fine powderand pressed to form an integral compact to facilitate handling.

The phosphor composition will be in single or multiple phase and havethe general formula E M, (A0 The anion within the host matrix isrepresented as A0, and may be either the phosphate or vanadate ion. Erepresents a rare earth element such as yttrium, scandium, lanthanum,gadolinium or lutetium, that will not act to quench the accumulations ofexcitation energy within the phosphor. The transition elements arerepresented by M and normally replace some of the yttrium or other rareearth within its crystal structure. Generally, none of the values of .x,y or 2 will exceed one of the other values by a ratio of more than 10:1.It will be seen that iron, cobalt, nickel, manganese, chromium andcopper are effective host materials within a luminescent phosphor. Itcan also be expected that other elements within groups VIIB, VIII and [Bof the periodic table cited above will effectively act as phosphor hostmaterials in conjunction with the rare earth phosphates or vanadatesmentioned.

Typical phosphor host materials have been prepared with the followingstoichiometric compositions: Mn 04 0.s o.2 4 2 3( 04 2 a( 04 Cu(PO.,)and Y -,Cu(PO The particular stoichiometric composition for theyttrium-ion phosphate given above has been found to be the optimum forproducing the most intense lanthanide luminescence for those components.

It has also been found that the lanthanide luminescence can be enhancedby the introduction of a nucleating agent to promote the crystalformation. In the case of the iron-containing phosphor, a particularlysuitable agent is sodium pyrophosphate (Na P O This agent is preferablyadded in sufficient amount to provide a mole to mole ratio with iron inthe stoichiometric composition Y., ,,Fe PO The luminescence in thephosphor host matrix is provided by trace amounts of lanthanides. Themore effective fluorescing lanthanide elements include Ce, Pr, Nd, Pm,Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb. In addition gadolinium will produceultraviolet spectral lines in many host matrices; however, itsluminescence is quenched in iron. It will also be seen that manganeseproduces spectral emissions when this element is included in thephosphor.

In employing the method of the present invention to detect and determinelanthanides in copper, iron and various transition elements, a sample ofthe metal is dissolved in an acid solution and the desired phosphorprepared as described. The phosphor is exposed to an X-ray, gamma-ray,or other electromagnetic radiation source to excite any lanthanideadditives or impurities that are contained. The resulting luminescenceis resolved into spectral lines and compared with spectra of knowncompositions to determine the lanthanides contained within the sample.When manganese is present, it too can be determined by this method.

Suitable X-ray sources and spectrometers as well as detailed techniquesfor their use are well known in the art and are illustrated in theabove-cited article and in Analytical Applications of X-ray ExcitedOptical Fluorescence Spectra: The Internal Standard Principle" byDeKalb, Fassel, Taniguchi and Saranathan, Analytical Chemistry, Vol. 40,pp. 2082-2084, Dec. I968. For example, the phosphor is placed in ashielded chamber provided with a shuttered port for exposure to an X-raysource. A quartz lens is appropriately positioned in the chamber forfocusing the sample luminescence onto the entrance slit of a gratingspectrometer. The X-ray source can be commercially-available,tungsten-target X-ray tube designed to operate at an electricalpotential of 50 KV. The spectrometer can be 25 cm focal length with aplane, diffraction grating and a fusedsilica-window photomultiplier(e.g. RCA 7268). The entrance and exit slits of the spectrometer are setat I microns and the spectrum may be scanned at the rate of about 200angstroms per minute as the phosphor is irradiated. The resultingphotocurrent is amplifled and recorded on a suitable strip chartrecorder.

The method of correlating experimental and calibration data in terms ofan internal standard is applicable in the instant case. A known butsmall amount of a lan thanide such as erbium is added along with the Y Oor other rare earth oxide. The concentrations of other lanthanides inthe sample are correlated in a graph or other means to the opticalintensity ratio of the lanthanide fluorescence to erbium fluorescence atcharacteristic peaks of each element. By treating data in this manner,many effects which would ordinarily influence luminescence, such as timeand temperature of heating, residual impurities and X-ray power drift,are satisfactorily diminished.

The following examples are offered to illustrate the phosphorcomposition of the present invention, its preparation and the method ofits use in determining lanthanides in transition elements.

STAINLESS STEEL EXAMPLE l Two 34-milligram samples of 303 stainlesssteel (C 0.l5% max., Mn 2% max., Si 1% max., Cr l7%-l9%, Ni 87o-I07c, S015% min., Fe balance) were electrolytically dissolved in separatesolutions of I07: HCIO, in CH COOH. To each solution was added I58 mg ofY O dissolved in l07r HNO in H Then a dilute nitric acid solutioncontaining Ce, Nd, Sm- O Eu o (M 0 Tb,0,, D 1 0, in a sufficient amountto equal I00 ppm of each lanthanide in the stainless steel was added toone of the solutions. For purposes of this application ppm refers toparts per million by weight. Both mixtures were evaporated to drynessand dehydrated at about 200C. To each dry mixture was added 370 mg of(NH HPO, and sufficient Na P O to be in a one to one mole ratio withiron. After thorough blending, the mixtures were maintained at 850C. for1 hour. The resulting phosphors were then exposed to an X-ray sourceconsisting of a Machlett DEG-50 X-ray tube with a tungsten targetoperated at a maximum input of 45 milliamperes at a potential of 50kilovolts. The luminescent spectra were recorded during the irradiation.

FIG. 1 illustrates the spectra obtained from these samples with theupper curve representing the sample including the lanthanides and thelower curve representing the dissolved stainless steel withoutlanthanide additives. Strong spectra lines appeared for maganese and forall of the lanthanides except gadolinium included in the doped sample.The lanthanide peaks in the lower curve are attributed to residualswithin the Y O These residual lanthanides can thereby be determined tocorrect the analysis of the other sample.

MISCHMETAL IN IRON EXAMPLE II A sample of low alloy steel containing asmall amount of mischmetal and a second sample of relatively pure ironpowder were dissolved and treated with Y O (NH HPO and Na P O insubstantially the same manner as described in Example I. Sufficient Y Oand (I\II-l,,) I-IPO were added to provide a stoichiometric compositionof Y Fe PO., which was previously found to be in optimum proportion forproducing lanthanide luminescence. The resulting X-ray luminescentspectra of the two samples are shown in FIG. 2 with the lower curvetaken from the pure iron sample and the upper curve taken from thesample of steel alloy including lanthanides. Again the peakscorresponding to terbium and other lanthanides in the lower curve areattributed to impurities in the yttrium. By comparing the spectra withthat of known samples, it was found that the steel contained 500 ppm ofNd, I40 ppm of Pr and 50 ppm of Ce.

In the above and other iron samples, a known amount, about 20 ppm oferbium, was added along with the Y O in order to introduce an internalstandard into the sample. Optical intensity ratios of other lanthanidesto erbium are plotted against concentration at the indicated wavelengthsas illustrated in FIG. 3. As mentioned above, the use of an internalstandard will satisfactorily compensate for many experminetal effects.Trace quantities of Ce, Pr, Nd and Sm as low as 5 ppm have beendetermined by this technique.

COBALT EXAMPLE III A sample was prepared containing 100 ppm of thelanthanides Sm, Eu, Gd, Td, and Dy in a Y Co (PO host. 0.87 gm of Co(NO.6 H O were intermixed with 0.23 gm of Y O and 0.53 gm of(NH HPOApproximately 0.73 ml of 0.1 nanogram/ml solution of each lanthanidewere added to the dry mixture which was then dehydrated and thoroughlyblended. The residue was next heated at 950C, l050C., and 1200C. withX-ray luminescent spectra curves produced after heating at eachtemperature. The strongest lines were found after the heating at 1050C.and are shown in FIG. 4.

Various other matrix hosts including those of stoichiometriccompositions Y Mn (PO Y Ni (PO and NaYCu(PO were prepared in essentiallythe same manner as disclosed in the above examples. Although notessential. it was found that the addition of sodium cations within thehost containing copper enhanced spectral emissions Sm, Eu, Gd, Tb and Dywere added into each of these host materials in sufficient quantities tobe equal to about 100 ppm of each named lanthanide in each phosphor. TheX-ray excited luminescent spectra for these materials are illustrated inFIGS. 5, 6 and 7.

It can be seen that a new phosphor composition is presented which isuseful in the detection and determination of lanthanides in steels andvarious transition metals. The determination method has beendemonstrated at levels of 5 ppm lanthanide impurities or additives. Thissensitivity is sufficient for most quality control procedures in theproduction of steel products. However, in nuclear reactor constructiononly steel products having lower concentrations of lanthanides aregenerally thought to be acceptable for good neutron economy. Therefore,the method at the demonstrated sensitivity may be used in this respectto identify those construction materials which clearly have too high alanthanide content for nuclear reactor use.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A luminescent phosphor composition comprising an iron-yttriumphosphate as a matrix host having a stoichiometric composition ofY,Fe,,PO where x is about 0.8 and y is about 0.2, said matrix hostincorporating a quantity of activator element in effective amount inexcess of 5 ppm by weight to produce de tectable luminescence on X-rayexcitation, said activator element selected from the group offluorescing lanthanides consisting of Ce, Pr, Nd, Sm, Eu, Tb, Dy and Er.

2. The composition of claim 1 wherein there is included a nucleatingagent consisting of Na P O within said matrix host in an amount of about1 to 1 mole ratio to the amount of said iron.

1. A LUMINESCENT PHOSPHOR COMPOSITION COMPRISING AN IRON-YTTRIUMPHOSPHATE AS MATRIX HOST HAVING A STOICHIOMETRIC COMPOSITION OF YXFEYPO4WHERE X IS ABOUT 0.8 AND Y IS ABOUT 0.2, SAID MATRIX HOST INCORPORATINGA QUANTITY OF ACTIVATOR ELEMENT IN EFFECTIVE AMOUNT IN EXCESS OF 5 PPMBY WEIGHT TO PRODUCE DETECTABLE LUMINESCENCE ON X-RAY EXCITATION, SAIDACTIVATOR ELEMENT SELECTED FROM THE GROUP OF FLUORESCING LANTHANIDESCONSISTING OF CE, PR, ND, SM, EU,TB, DY AND ER.
 2. The composition ofclaim 1 wherein there is included a nucleating agent consisting ofNa4P2O7 within said matrix host in an amount of about 1 to 1 mole ratioto the amount of said iron.